Gaseous plasma technology is a well known technique used for the fabrication of integrated circuits. More specifically, plasma technology has been used with great success for thin film etching, deposition of films onto wafers, photoresist stripping, etc.
Despite the widespread acceptance of plasma technology in the semiconductor industry, the use of plasma techniques continues to present a number of challenges. Notably, it has sometimes proven difficult to ensure uniform distribution of reactive gas over a semiconductor wafer during processing. It has been especially problematic to ensure uniform distribution of gas over the semiconductor wafer to accomplish photoresist stripping.
FIG. 1 is illustrative of a prior art process chamber which suffers from non-uniform reactive species distribution. Gas containing active species is passed through a central conduit 102 into a process chamber 104. The gas reacts with the surface of the semiconductor wafer 108 positioned on a platen 116 and then passes out through central exhaust conduit 106. The residence time of the gas in the chamber is typically short (e.g. a few milliseconds). At chamber pressures close to 1 torr and flow rates up to 5 standard liters per minute the Reynolds number is low and the flow is well into the viscous-laminar regime (e.g. Reynolds number "R"&lt;2000). The flow lines are highly ordered over the wafer surface, as illustrated by exemplary flow lines 110 and 112.
As shown in FIG. 1, the inlet 118 of the central conduit 102 is typically positioned directly above the middle portion of the semiconductor wafer 108. As such, the middle portion of the semiconductor wafer receives a disproportionate share of active species, which react and are consumed at the wafer surface. After reacting with the wafer surface, the central stream of gas (e.g. 110) flows radially across the wafer surface and then out through the exhaust conduit 106. This radial flow of gas across the wafer surface precludes outlying streams of gas (e.g. 112) from reaching the wafer surface. It is possible for the active species in these outlying streams of gas to diffuse across the radial flow lines. This diffusion is proportional to the temperature and pressure of the gas. Nevertheless, the short residence time of gas in the chamber usually prohibits a significant amount of diffusion. As a result, the wafer surface is etched at a disproportionate etching rate, e.g., the middle portion of the wafer is etched at a faster rate than the remote edge portion of the wafer.
Practitioners in the art have attempted to mitigate the above noted effect by employing a showerhead to distribute the active species over the semiconductor surface. As shown in FIG. 2, a showerhead 202 is placed between the central conduit inlet 118 and the semiconductor wafer 108. The showerhead typically consists of a plate with a plurality of orifices 220. Gas supplied through the central conduit passes through the plurality of orifices 220, thereby more evenly distributing the application of active species at the surface of the substrate 108.
Yet, in most cases, the prior art showerhead still fails to produce satisfactory uniformity in etching. Gas is eventually pumped through a single exhaust conduit 106. The flow of gas into the conduit 106 creates the same undesirable radial flow 210 of gas across the surface of the wafer, which blocks outlying streams of active species from reaching the wafer surface. Accordingly, the center of the wafer is etched at a faster rate than the rest of the wafer, although not necessarily to the same degree as in the apparatus shown in FIG. 1.